Laser apparatus for optical modulation transfer

ABSTRACT

There are disclosed arrangements for transferring optical information from one optical beam of a first frequency to another optical beam of a second frequency within a laser apparatus, particularly for the cases in which the laser transitions corresponding to the two frequencies are independent or very strongly competitive. The arrangements are based upon the use of a broadband gain medium, such as a dye laser medium, to modify the intensity relationship between two or more independent transitions or two or more competing transitions of a single laser medium.

307- HA5 xo 3,731,223

United States Patent L 91 1 3,731,223

Johnston, Jr. 1 May 1, 1973 [54] LASER APPARATUS FOR OPTICALKliot-Dashinski, Optics & Spectroscopy, v01. 29, no.

' MODULATION TRANSFER 1, July, 1970, pp. 75-8.

Kliot-Dashinski, Soviet Physics-Tech. Physics, vol.

[75] Inventor: gveillbglllj Dexter Johnston, Jr., l-lolm- 15, no. 7January 1971,

[73] Assignee: Bell Telephone Laboratories, lncor- PrimaryExaminer-Ronald ibe t g d Murray Hill NJ Assistant Examiner-R. J.Webster AnorneyR. Jv Guenther et a1. [22] Filed: Sept. 23, 1971 21Appl.No.: 183,076 ABSTRACT There are disclosed arrangements fortransferring 0p- 52 U.S.Cl. ..331/94.5, 250/199, 307/883 Iioalinformation from one optical beam of a first 511 int. Cl .11015 3/20,HOls 3/10, H018 3/09 frequency to anorhor optical beam of a second [58]Field of Search ..331/94.5; 29/199; frequency Within a laser apparatus,particularly for the 307/883 cases in which the laser transitionscorresponding to the two frequencies are independent or very strongly[56] Reerences Cited competitive. The arrangements are based upon theuse of a broadband gain medium, such as a dye laser UNITED STATESPATENTS medium, to modify the intensity relationship between two or moreindependent transitions or two or more 3,333,101 7/1962 Bell ..250/83.3competing transitions of a single laser medium.

OTHER PUBLICATIONS Kliot-Dashinski, Optics & Spectroscopy, vol. 26, no.6, June, 1969, pp. 538541 '6 Claims, 4 Drawing Figures ELECTRICALEXCITATION I71 SOURCE Li 23 I3 M MODULATEDi SIGNAL 1 w x l ?l. LASERHAVING 1a l, ISOLATOR AT LEAST BEAM TWO LINES WMST OUTPUT COUPLES TWO xOSCILLATING 2 LINES PATENTED 1 3.731 .223

sum 10F2 ELECTRICAL EXCITATION |7-'\ souncz 1a MODULATED SIGNAL SOURCE\V I I9 LASER HAVING x ISOLATOR AT LEAST BEAM TWO LINES WAST OUTPUTCOUPLES TWO A OSCILLATING 2 LINES FIG. 2 v

ELECTRICAL EXCITATION souncz 37-3 COUPLES TWO OSCILLATING (QPTIONAL xOUTPUT) 2 L ES LASER HAVING 38 OUT AT LEAST 3 TWO LINES PATENTED W H9753. 731 .223

SHEET 2 OF 2 FIG. 3

I5 ISOLATOR 12 22 I I4 MOD. a SIGNAL I SOURCE LASER HAVING I AT LEASTTWO LINES 22 2 l8 /OUTPUT 52\XI l 3 53K \55 suBsTR/ITE ova-00 m FILMUPLING FIG. 4

XENON LASER x= osasz 4 NET GAIN P0 PRESSURE LASER APPARATUS FOR OPTICALMODULATION TRANSFER BACKGROUND OF THE INVENTION This invention relatesto the transfer of information from one optical carrier of a firstfrequency to a second optical carrier of a second frequency.

While lasers have frequently been proposed as being useful for opticalcommunication systems, one of the obstacles to practical systems of thistype is the lack of an adequate variety of high performance devices,including sources, modulators, amplifiers and detectors at any onefrequency. In order to take optimum advantage of the various highperformance optical components now available for use in a communicationsystem, it is desirable to be able to shift the frequency of the opticalcarrier at various points in the transmission system. Moreover, thisfrequency shifting should be achievable relatively efficiently in arelatively simple apparatus.

While various proposals for such frequency shifting have been made,nearly all of them are either highly inefficient or unduly complicated.For example, demodulation of the first optical carrier and use of thebaseband signal to modulate a new optical carrier introduces problemsboth at the detection stage and at the remodulation stage which shouldbe avoided. The

complexity of such a system is objectionable. Other schemes which do notinvolve demodulation, such as parametric mixing or the use of two photonabsorption, as disclosed in the copending patent application of H. P.Weber, Ser. No. 142,680, filed May 12, 1971, and assigned to theassignee hereof, are relatively inefficient and seem impractical formany commercial applications.

SUMMARY OF THE INVENTION According to my invention, I provide thedesired modulation transfer, or optical frequency shifting of theoptical carrier, in an apparatus in .which a first laser active mediumamplifier an input modulated beam via a first one of two radiativetransitions in said medium. The amplified beam then pumps a dye lasermedium which couples the two transitions to produce oscillations at thefrequency of the second transition because of optical feedback from thesecond medium to the first medium.

The coupling provided by the second laser medium, specifically the dyelaser medium, permits the use of a wide variety of materials as thefirst laser medium. In fact, the dye laser medium can couple thetransitions with a strength providing a substantial degree of modulationtransfer.

According to a subsidiary feature of my invention, the modulated firstoptical carrier is coupled into the apparatus through an isolator or athree-port optical circulator, the latter arrangement having theadvantage that an amplified beam at the first frequency can be obtainedfrom one of the circulator parts while the frequency shifted carrier canbe extracted either from another part of the laser apparatus or, ifdesired, collinearly with the amplified beam at the first frequency.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of myinvention will become apparent from the following detailed description,taken together with the drawing, in which:

FIG. 1 is a partially pictorial and partially block diagrammaticillustration of a first embodiment of my invention;

FIG. 2 is an illustration of a modification of the embodiment of FIG. 1employing a three-port optical circulator;

FIG. 3 is an illustration of a further modification of the embodiment ofFIG. 1 employing a broadband active thin film; and

FIG. 4 shows curves useful in explaining the operation of theembodiments of FIGS. 1 through 3.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS The specific embodiments of myinvention can best be understood with the following backgroundinformation. The majority of laser gain media other than semiconductorcompounds are capable of providing laser action at several differentwavelengths, when placed in an appropriate optical cavity and excitedappropriately for the desired wavelength. For some of these transitionsin certain media, simultaneous laser operation is possible on two ormore wavelengths; that is, the appropriate optical cavity and excitationconditions mentioned are not mutually exclusive. Two such transitions ina gain medium are said to be independent, if they share neither uppernor lower laser level; in cascade, if the upper level of the one is thelower level of the other; or in competition, if they share the sameupper level or the same lower level.

The light output at different wavelengths is unrelated, if thetransitions are independent. That is, suppressing the output at onewavelength by external means, such as introducing wavelength-selectiveabsorption into the cavity, does not affect the light output at theother wavelength. If the transitions are in cascade, the light outputsat the different wavelengths vary together, so that oscillation on onetransition increases the gain and thus tends to increase the lightoutput at the other transition. When two transitions are in competition,simultaneous operation is unstable and one or the other will dominatecompletely in the light output.

The introduction of an organic dye solution into the optical cavityprovides a mechanism by which the nature of the transition coupling maybe modified. Thus, if we have two simultaneously oscillating independenttransitions, they may be coupled by choosing a dye which absorbs at theshorter wavelength and fluoresces at the longer wavelength, and whichhas such other qualities (which are well known from dye lasertechnology) as will result in gain at the longer wavelength. Thus, inthis example, the shorter wavelength transition experiences less, andlonger wavelength greater, gain than when the dye is not present. Withan appropriate dye density, and with appropriate adjustment of opticalspot size in the dye cavity, gain is thus switched from the shorter tothe longer wavelength transition. With regard to coupling, they appearto be partially in cascade, in that suppression (enhancement) of theshorter wavelength oscillation reduces (increases) the gain for thelonger wavelength. There is no reverse coupling. That is, there is noeffect on the shorter wavelength by suppression or enhancement of thelonger wavelength, although there would be an effect in true cascade.

As another example, if we have two transitions which compete stronglyfor gain but which could oscillate simultaneously were it not for thecompetitive coupling, the competition can be reduced from an all one orall the other case to a case in which a variable finite ratio of lightoutput may be obtained. This reduction in competition is accomplished byselecting the optical cavity so that oscillation is obtained on theshorter wavelength transition in the absence of the dye. When the dye isadded, again with appropriate density and in a position in the cavitywith appropriate optical spot size, gain is again switched from theshort wavelength transition to the long wavelength transition, bringingit above threshold. It cannot now extinguish the short wavelengthoscillation, however, since the additional gain switched to the longwavelength transition is proportional to the power in the cavity at theshort wavelength transition. The ratio at equilibrium may be adjusted byvarying the total excitation, dye density, spot size in the dye cell,and initial absorptive or transmissive discrimination (if any) againstthe longer wavelength or in favor of the shorter.

The embodiment of FIG. 1 provides modulation transfer from the carrierfrequency of the modulated signal source 21 to a second.carrierfrequency, a portion of which can be extracted through the end mirror 18of the laser apparatus. This apparatus illustratively couples twoindependent oscillations, which can also be referred to as radiativetransitions or lines of the xenon laser 12. The laser 12 is disposed ina folded optical resonator including the back mirror 13 which is highlyreflective at A, and 90 percent reflective at A,, the highly reflectivebroad band oblique-incidence mirror l4 and the output mirror 18 which is90 percent reflective at A, and highly reflective at A,. Specifically,A, is 5,352 Angstrom units and A, is 5,956 Angstrom units; thesewavelengths are the wavelengths of two independent laser transitions inthe xenon laser 12 when subjected to a direct current discharge from asource 17 connected between anode l and cathode 16.

Also disposed in the folded optical resonator is the dye cell 19, whichhas Brewster-angle entrance and exit surfaces forming the Brewsterangles in the same plane as the plane of the folded optical resonator.The astigmatism of these Brewster-angle surfaces of dye cell 19 ispartially compensated by the choice of angle between the axes of the twosections of the folded optical resonator, as taught in the copendingapplication of A. Dienes et al., (Case 3-5-21-6) Ser. No. 154,087, filedJune 17, I971, and assigned to the assignee hereof.

The dye cell 19 illustratively includes rhodamine 6G dye in a methanolsolution. The particular degree of coupling of the laser transitionscould be modified by mixing the rhodamine 6G dye with other dyes such asrhodamine B or by using other appropriate dyes.

Further details of the embodiment of FIG. 1 are as follows:

The reflectivity and radii of mirrors l3, l4 and 18 are selected toprovide oscillation of the laser 12 at 5,352 Angstrom units, as well assubstantial reflectivity at 5,956 Angstrom units, a wavelength ofoscillation available in a xenon laser. The radii of reflectors l4 and18 provide the waist of the beams in the center of dye cell 19, throughwhich the dye is flowed to avoid bleaching and damaging effects.

The xenon discharge tube 12, in some of my preliminary experiments, waspulsed electrically so that optical pulses of about 250 nanosecondsduration were obtained 10-20 times per second. Nevertheless, theseparameters are not believed to be critical. Pairs of continuous-wavelines are available in other suitable lasers.

The operation of the embodiment of FIG. 1 can be understood as follows:The dependence of the gain on total gas pressure in tube 12 for the5,352 Angstrom and 5,956 Angstrom transitions in tube 12 is illustratedby curves 4] and 42, respectively, of FIG. 4, in the absence of dye cell19. Note that there are regions of curves 41 and 42 that indicate thateither transition will oscillate alone, specifically, to the left andright of the region of overlap, and a region of overlap whereindependent oscillations on both transitions occur simultaneously. Ifthe pressure is adjusted to the operating point on curve 41 havingpressure P,,, the 5,352 Angstrom line is oscillating strongly and the5,956 Angstrom line is somewhat below threshold. Next, the concentrationof the dye in cell 19 is gradually increased until the 5,956 Angstromunit transition is brought above threshold and its coherent radiation istumed on." Concurrently, the power at 5,352 Angstrom units decreases. Adye concentration of the rhodamine 6G corresponding to one-wayabsorptive loss of 17 percent at 5,352 Angstrom units was found to beoptimum to maximize the output at 5,956 Angstroms for the particulararrangement of FIG. 1 having the gains and losses present in my earlyembodiment.

The dye cell 19 acts to put the two independent transitions of laser 12effectively in cascade; that is, if the intensity of the beam at thefrequency of the first optical carrier varies, the intensity of the beamat the second optical frequency will similarly vary. In order for alinear modulation transfer to be achieved, the reflectortransmissivities and the dye concentration in cell 19 are adjusted sothat the longer wavelength oscillation is just above threshold and theshorter wavelength oscillation is simultaneously just above thethreshold and continuing to oscillate without any signal from modulatedsignal source 21.

Now let us recall that reflector 18 is totally reflecting for theshorter wavelength A, oscillation; and reflector 13 is totallyreflecting for the longer wavelength A, oscillation. Reflector 18 ispartially transmissive at A, to couple a portion of thefrequency-shifted modulated optical beam out for utilization; andreflector 13 is partially transmissive for the shorter wavelength A,oscillation, since it is necessary to couple in to the laser apparatusthe original modulated beam at wavelength A,.

Specifically, the optical modulated carrier at wavelength A, is coupledinto the laser resonator through an isolator 22 and the lens 23, underconditions insuring that the oscillation in the resonator formed byreflectors l3, l4 and 18 at wavelength A, is locked to the input signalat wavelength A,. This locking may be a phase lock for modulationincluding phase modulation, although pure phase modulation cannot betransferred by this scheme. The locking may be an amplitude lock for anamplitude modulation or even a spatial lock to the oscillation asdescribed in U.S. Pat. No. 3,576,502, issued Apr. 27, 1971. By virtue ofthe dye coupling, the gain at wavelength A, is modulated in time andspace in proportion to the signal at A and thus the output beam atwavelength A, is also modulated in proportion to the signal at A Theforegoing scheme is obviously generalizable to other transitions inxenon or other gas discharges and to other types of gain media such ascrystalline or liquid materials. It is only necessary that the shorterwavelength transition lie within the absorption (or pump band) of thesecond laser medium, i.e., that of cell 19, and that the longerwavelength transition of the first laser medium lie within the gainbandwidth of the second laser medium. This fact can be appreciated froma perusal of the curves of FIG. 4, as explained above.

It should be stated here that the embodiment of FIG. 1 will also workwhen the laser 12 is a laser that provides strong competition of the twotransitions such as occurs in a neodymium laser at wavelengths of )t I06and 1.3 micrometers. In this case, the second laser medium, i.e., cell19, shifts gain from the first transition to the second transition andprevents the vigorous competition between the two transitions fromextinguishing the second, and usually weaker, transition. It will beappr'eciated that without the dye cell 19, only one or the othertransition will oscillate, since oscillation on the one transitionreduces the gain for the other. Nevertheless, with the dye, thecompetitive coupling can be reduced to a level at which both transitionsare allowed to oscillate simultaneously.

Since an antiphase modulation would normally be expected for such amodified embodiment, the laser apparatus would normally be adjusted sothat the longer wavelength k, transition is oscillating strongly in theabsence of the input signal; but the shorter wavelength transition isalso oscillating at least weakly. In fact, an injected signal at eitherA, or A, will produce an antiphase modulation of the output at the otherwavelength.

In order to obtain an output at either the longer or the shorterwavelength, isolator 22 of FIG. 1 should be replaced by the three-portoptical circulator 30, shown in FIG. 2 All of the other components ofFIG. 2 are similar to .the analogous components of FIG. 1 with theexception of the initial adjustment of the A, oscillation and thepresence of strong competition between the two transitions in theabsence of dye cell 39.

The three-port optical circulator 30 is of known type, such as disclosedin U.S. Pat. No. 3,267,804, issued Aug. 23, I966 to J. F. Dillon, .Ir.It will transfer the A, signal from port 1 to port 2 when the signal istraveling clockwise from port 1 to port 2 and will transmit a signal atA, from port 2 to port 3 when that signal enters port 2 upon returningfrom the laser resonator and lens 23. The amplified signal at wavelengthA, is extracted from port 3. Optionally, the signal at k, can also beextracted from port 3, if desired for some applications. For thispurpose, mirror 33 would be selected to be partially transmissive at M,instead of mirror 38. Mirror 38 would then be made highly reflective atA,, as well as at A Even for some other laser system in which cascadedtransitions exist, the use of the additional gain medium 19 or 39provides a mechanism for increasing the efficiency of the in-phasetransfer of modulation for an injected signal at A since the gain at A,is being increased not only by virtue of the cascade, but also by thedye gain. Since the transfer characteristics of the cascade coupling aredetermined by the various radiative and nonradiative lifetimes of theupper, common, and lower cascade levels, which are not susceptible toindependent adjustment for optimization, the second gain medium providesan adjusting mechanism for optimizing the particular modulation transferdesired.

The advantages of an organic dye as compared to other second gain mediain this system reside in its broadband-absorption and broad gainbandwidth.

It should, of course, be understood that a variety of organic dyes areavailable which will provide the absorption band and emission bandsimilar in shape to those shown by curves 41 and 42 of FIG. 4 butmatched in wavelength to the transitions of the lasers 12 or 32.

In the embodiment of FIG. 3 all components labeled the same as in FIG. 1are essentially the same as in the embodiment of FIG. 1. The principaldifferences from the embodiment of FIG. 1 are that the dye cell 19 hasbeen replaced by the thin film assembly, including prisms 51 and 54,substrate 53 and thin film 59.

Illustratively, the substrate 53 is a low index (1.47) PYREX glass andthe film 59 is a polyethylene film, index 1.55, doped with rhodamine 66such as was used in the embodiment of FIG. 1. The film may be applied inliquid form to substrate 53 by dipping, spraying or painting. The inputand output coupling are provided by the prisms 51 and 54, which areseparated from film 59 by a small gap 52, as taught by P. K. Tien inU.S. Pat. No. 3,584,230, issued June 8, l97l. A substantial gain inlight intensity in the thin film is achieved so that the dyeconcentration need not be as great as in cell 19 of FIG. 1, bettercooling is obtained so that the rate of destruction of the dye is lowerand, in addition, the focusing requirements are substantially reduced.It will be noted that output mirror 18 closes an optical oscillar torwhich extends through the prisms 51 and 54 and the intervening portionof thin film 59. As damage of the dye in thin film 59 occurs, it ismerely necessary to displace the thin film 54 laterally under the prismsso that the oscillation path passes through a fresh portion of the dye.

I claim:

1. Apparatus comprising a first laser active medium, means for pumpingsaid medium to invert the populations of two transitions of said medium,means for admitting to said first laser active medium an input modulatedbeam of a first frequency matching the frequency of radiation from afirst one of said transitions, a second laser active medium disposed tobe buffered by said first medium from said input beam and to interceptradiation from said first active medium, said second medium absorbingradiation at said first frequency to produce population inversionyielding laser gain over a band of frequencies including the frequencyof radiation of the second transition, and means including an opticalresonator disposed about both said first and second media for providingoptical feedback from said second medium to said first medium andstimulated emission of radiation on at least one of said transitionsfrom at least said first medium, said first and second media togetheremitting radiation at said first and second frequencies with intensitieshaving respective one-to-one correspondences to the intensity of theinput beam, said apparatus thereby providing a substantial degree ofmodulation transfer to radiation at said second frequency.

2. A laser according to claim 1 in which the two transitions of thefirst laser active medium are independent transitions, and the secondlaser active medium has an absorption for a portion of the radiation ofthe shorter wavelength transition and provides gain for the longerwavelength transition to couple said two transitions to a degree causingboth transitions to oscillate near threshold in the absence of an inputsignal.

3. A laser according to claim 1 in which the two transitions of thefirst laser active medium are competing transitions, said resonatorfavoring relatively strong oscillations of the second transition, thesecond laser active medium being effective to yield a stable antiphasemodulation of an output beam of the frequency of the second transitionin response to the admitted input modulated beam at the first frequency.

4. A laser according to claim 1 in which the admitting means includes anoptical isolator to prevent the admitted input beam from propagating inreverse therethrough.

5. A laser according to claim 1 including an optical circulator havingone port from which an amplified beam at the first frequency can beobtained.

6. A laser according to claim 5 in which the resonator includes areflector disposed between the optical circulator and the first medium,said reflector being partially transmissive at both of the first andsecond frequencies to permit the extraction of modulated radiation atthe second frequency from the one port collinearly with the amplifiedbeam at the first frequen-

2. A laser according to claim 1 in which the two transitions of thefirst laser active medium are independent transitions, and the secondlaser active medium has an absorption for a portion of the radiation ofthe shorter wavelength transition and provides gain for the longerwavelength transition to couple said two transitions to a degree causingboth transitions to oscillate near threshold in the absence of an inputsignal.
 3. A laser according to claim 1 in which the two transitions ofthe first laser active medium are competing transitions, said resonatorfavoring relatively strong oscillations of the second transition, thesecond laser active medium being effective to yield a stable antiphasemodulation of an output beam of the frequency of the second transitionin response to the admitted input modulated beam at the first frequency.4. A laser according to claim 1 in which the admitting means includes anoptical isolator to prevent the admitted input beam from propagating inreverse therethrough.
 5. A laser according to claim 1 including anoptical circulator having one port from which an amplified beam at thefirst frequency can be obtained.
 6. A laser according to claim 5 inwhich the resonator includes a reflector disposed between the opticalcirculator and the first medium, said reflector being partiallytransmissive at both of the first and second frequencies to permit theextraction of modulated radiation at the second frequency from the oneport collinearly with the amplified beam at the first frequency.